STATE &ANT1 
 
 Waited States Department of Agriculture 
 Agricultural Research Administration 
 Bureau of Entomology and Plant Quarantine 
 
 COMPARISON OF FOUR INSECTICIDES, INCLUDING- DDT, IN MINERAL OIL 
 TOR CONTROL OP THE CORN EARWORM IN DENT SEED CORN 
 
 By R. A. Blanchard and A. P. Satterthwait, 
 Division of Cereal and Forage Insect Investigations 
 
 Inbred lines of com used in making commercial single crosses 
 are often severely damaged by the corn earworm ( Heliothis armigera Hbn.), 
 as well as by diseases that develop in the ears following earworm attack. 
 The extent of such damage is dependent upon the susceptibility of any 
 particular line to earworm and disease, and also upon the number of 
 larvae entering the ear and the time of their entrance. 
 
 In Illinois the earworm may cause damage to corn either in the 
 roasting-ear stage or in the maturing ears. Diseases are more often 
 associated with the first type. Loss from the second type is due to 
 feeding by the larvae on the outer parts of the kernels, or to their 
 burrowing beneath the kernels so that they are lost in harvesting. 
 Kernels with any appreciable surface feeding must be discarded. 
 
 The method recommended by Barber and coworkers ( l-U) and 
 Davidson for the control of the earworm in sweet corn grown for 
 roasting ears or for seed is the injection of mineral oil containing 
 pyrethrum, dichloroethyl ether, or styrene dibromide into the silk mass 
 Just above the tips of the ears at about the time the silks begin to 
 turn brown. The degree of control obtained by this treatment in dent 
 corn being grown for seed has been questionable. Tests comparing DDT 
 (l-trlchloro-2,2-bis(p-chlorophenyl) ethane) with the other insecticides 
 were therefore made at Decatur, 111., in 1$UU, in an effort to obtain a 
 greater reduction of worm damage. It was already known, from experi- 
 ments conducted by Ivy (6), Johnson (j) t and Stevenson, Sheets, and 
 Breazeal* (g), that DDT Ts highly toxic to the corn earworm. It was 
 also known "that the degree of control from the injection of oil plus an 
 insecticide depends on the length and tightness of the husks as well as 
 on the amount of oil injected. On the other hand, it was known that 
 excessive amounts of oil prevent development of kernels on the tips of 
 the ears. The experiments were therefore designed to take all these 
 factors into account* 
 
Small-Plot Experiment 
 
 Eight inbred lines, four long-husked and four short-husked, 
 with husks ranging from loose to very tight, were planted in triplicate. 
 All the plots were planted and cared for by the Bear Hybrids Com 
 Company on its seed-production farm at Decatur, On the same farm the 
 long- and short-husk conditions were simulated in one inbred line, U.S. 
 187-2, "by planting it in duplicate and clipping "back the husks of one 
 planting to the tips of the ears. One replication of all eight lines 
 was destroyed "by chinch "bugs ( Blissus leucopterus (Say)). 
 
 Two highly refined mineral oils were used, Bayol (80-90 seconds 
 Saybolt) and Super la No. 13 (120-125 seconds Saybolt). Each insecti- 
 cide was tested in both oils, in the following concentrations: DDT 2 
 percent, pyre thrum extract (0,2 percent pyrethrins), dichloroethyl 
 ether 2 percent,- and styrene dibromide 1 percent. They were applied at 
 two dosages, 0.6 and 1.2 ml. of solution per ear. 
 
 Each plot was 6 hills long, with a maximum of 3 plants per hill. 
 The ears of 3 hills were treated with the insecticide dissolved in Bayol, 
 and the others with the insecticide in Superla No. 13. Th© number of 
 ears in each replication ranged from U to 9» with an average of 6.3. As 
 is shown later, the type of oil did not appear to influence the results, 
 and thus the average size of the sample in each replication that could 
 be used for comparing the various dosages was increased to 12.6 ears. 
 
 The corn was harvested at the normal time for gathering seed. 
 Ears from each plot were examined to determine the number of immature 
 kernels per ear damaged by the earworm and disease, and the number of 
 mature kernels per ear damaged by the earworm. The effect of DDT on 
 germination of the seed was also determined. Table 1 summarizes the 
 average percentage of infested ears per plot per inbred and the average 
 loss of kernels on all inbred lines in each treatment and in the un- 
 treated checks. It also gives the combined averages of both oils and the 
 two rates of treatment. In table 2 the results obtained from the four 
 insecticides is. both oils and at two rates of treatment are averaged 
 according to inbred line and husk type in order to show the effects of 
 length and tightness of husk on control. 
 
Table 1. —Effect of injection of insecticides in mineral oil into silks 
 
 corn for control of the corn earworm. Small-plot experiment, dosages 0,6 
 and 1.2 ml. of oil solution per ear, 19^4 
 
 
 
 
 Estimated 
 
 kernels lost r>er ear due to— 
 
 Living larvae 
 
 Treatment 
 
 Ears infested 
 
 Earworm and disease 
 in green corn 
 
 Earworm in 
 maturing corn 
 
 per ear at 
 
 harvests/ 
 
 0.6 ml. 
 
 
 
 1.2 ml. 
 
 0.6 ml.j 1. 
 
 2 ml. 
 
 0.6 ml. 
 
 1.2 ml. 
 
 0.6 ml^"l.2 ml 
 
 
 Percent 
 
 Percent 
 
 Number Number 
 
 Number 
 
 Number 
 
 Number Number 
 
 DDT 2 
 
 Bayol 
 Superla 
 
 16.7 12.} 
 17.2 ll.U 
 Av. lU.U 
 
 2.4 
 3.1 
 
 2.7 
 
 2.5 
 2.6 
 
 0.04 0.1 
 
 
 
 .Oh 
 
 0.0 O.lo 
 .025 
 
 .01 
 
 Pyre thrum: 
 Bayol 
 Superla 
 
 49.7 ^9.3 
 51.1 47.0 
 Av. 3 
 
 12.2 
 14.1 
 
 11.0 
 
 8.7 
 8.8 
 
 1.0 
 .9 
 
 1. 
 
 1.8 
 .7 
 
 1 
 
 .36 .25 
 
 .25 .12 
 
 .24^ 
 
 Dichloroethyl ether: 
 Bayol 68.1 58.1 
 
 Superla 67.6 58.0 
 Av. 63.O 
 
 15.1 
 12.0 
 
 11.1 
 
 8.8 
 8.5 
 
 1.4 
 .5 
 
 l.U 
 
 1:1 
 
 .295 .45 
 .292 »25 
 
 • 32 
 
 Styrene dibromide: 
 Bayol 59*2 
 Superla 6l.2 49.3 
 Av. 51.2 
 
 35.0 
 49.' 
 
 Checks (untreatecO 92 
 
 10.7 
 16.4 
 
 9.7 
 19.0 
 
 6.3 
 
 .5 
 1.0 
 
 1.0 
 2.3 
 
 0.6 
 1.7 
 
 .255 .?4 
 
 .248 .20 
 .24 
 
 .35 
 
 If Average represents larvae on one replication. 
 
 2 J An average of 106.9 ears of each inbred line examined. There were 4 check 
 plots in each replication. 
 
- U - 
 
 Table 2.— Effects of length and tightness of the husk covering of dent corn on 
 control of the com earworm "by injection of insecticides in oil into the 
 silk. Average of all ears injected with the four insecticides in "both 
 oils at "both dosages. 
 
 Type of husk 
 coverage 
 
 Inbred lines 
 
 Infested ears 
 
 Kernels damaged by ear worm 
 and disease 
 
 
 
 Treated 
 
 Untreated 
 
 Reduction 
 
 Treated 
 
 Untreated 
 
 Seduction 
 
 
 
 Percent 
 
 Percent 
 
 Percent 
 
 Number 
 
 Number 
 
 Number 
 
 Long, loose 
 
 U.S. 187-2 
 111. Eh 1/ 
 AvT 
 
 61.3 
 28.0 
 
 97.7 
 98.2 
 98.0 
 
 37.3 
 71.5 
 5U.H 
 
 17.2 
 H.O 
 10.6 
 
 39.1 
 15.5 
 27*3 
 
 56.0 
 7^.2 
 65.1 
 
 Short, loose 
 
 Ind. 38-11 53-3 
 111. MiU 39.7 
 U.S. 187-2 2/ 68.0 
 Av. " 53-7 
 
 86.9 
 81. U 
 96.U 
 88,2 
 
 3S.7 
 51.2 
 29.5 
 39.8 
 
 10.1 
 6.6 
 
 21.7 
 12.8 
 
 20.1 
 15.0 
 35.6 
 
 23.6 
 
 H9.8 
 56.0 
 39.0 
 U8.3 
 
 Long, tight 
 
 lans. Eh 
 
 6.U 
 
 93.5 
 
 93.2 
 
 .7 
 
 11.6 
 
 9U.0 
 
 Long, moder- 
 ately tight Ind. Wf9 
 
 1*.5 
 
 87.1 
 
 U8.9 
 
 7.1 
 
 18. h 
 
 61.U 
 
 Short, moder- 
 ately tight 111. Hy 
 
 55.2 
 
 97.3 
 
 ^3.3 
 
 10.0 
 
 20.6 
 
 51.2 
 
 Average, 
 
 all lines 
 
 W.G 
 
 92.3 
 
 51.7 
 
 9.7 
 
 22,0 
 
 55.9 
 
 1/ Although classed as loose-husked, the husk sticks to the ear surface in places 
 "" until maturity. 
 
 2 J Husks clipped back— %e tips of ears. 
 
 Commercial-Scale Experiment 
 
 The Bear Hybrids Corn Company also treated "by injection the ears on 
 two rows in each of two commercial foundation seed fields with 2 percent of 
 DDT in Superla No. 13 oil and on two other rows with 3 percent of dichloro- 
 ethyl ether in the same oil. Two rows in each field were left untreated as 
 checks. The treatments were made "by the regular crew in the course of 
 their application of dichloroethyl ether in oil to these fields. The solu- 
 tions were injected at the rate of 0,75 nl« P© 1 * ear* 
 
 At harvesttirae five 25- to 30-ear samples were taken from each'' area 
 and examined in the same manner as in the small-plot experiment. In addi- 
 tion the company processed the seed from each area separately, and deter- 
 mined the relative amounts of labor involved in sorting the ears, as well 
 as the approximate amounts of damaged corn. The results are shown in 
 table 3. 
 
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- 6 - 
 
 Discussion 
 
 DDT was the only insecticide which protected the corn until 
 harvesttime. DDT in mineral oil gave almost complete control of 
 the com earworn in "both the green and the maturing stages of the 
 corn ears in both experiments. Some of the slight damage shown 
 undoubtedly resulted from larvae already established before the 
 insecticide in oil was applied. In the small-plot experiment most 
 of the damage in the DDT plots occurred in one inbred line, U.S. 
 187-2. Pyrethrum, styrene dibromide, and dichloroethyl ether were 
 all less effective than DDT. Styrene dibromide ranked second to 
 DDT. 
 
 In the small-scale experiment the 1.2-ml. dosage was, in 
 general, superior to the 0.6-ml. dosage. The greatest increase in 
 control resulting from the larger dosage occurred with styrene 
 dibromide in long, loose-husked lineB such as U.S. 187-2. None of 
 the insecticides adversely affected germination. 
 
 The 0.6-ml. dosage did not appear to reduce the development 
 of the tips of the ears appreciably, whereas the 1.2-ml. dosage 
 caused some loss of tip kernels, especially in the tight-husked line 
 Kansas tk» This loss is thought to have been due to the oils, as 
 none of the insecticides tested appeared to retard kernel develop- 
 ment. The dosage of the solution should be limited to 0.6 ml. in 
 very tight-husked lines and increased in lines with loose husks. The 
 best control was obtained in the long, tight-husked line Kh. The 
 reduction in numbers of kernels damaged was somewhat greater in the 
 long than in the short, loose-husked lines. Tightness of husk seemed 
 more important than length for a high degree of control. There did 
 not appear to be any significant difference in the control obtained 
 with the two oils. 
 
 In 19UU by far the greatest saving in corn resulted from 
 prevention of the diseases that enter the ears following earworm 
 attack. This does not apply to those diseases that enter the ears 
 through the shank. 
 
 Of the four insecticides tested, DDT appears, at present, to be 
 the most ideally suited for use in controlling the earworm in dent 
 corn grown for seed. This insecticide produced no visible effects on 
 the operators, whereas styrene dibromide is very irritating to some 
 persons* 
 
- 7 - 
 
 Literature Cited 
 
 (l) Barter, G. W. 
 
 I9U2. Mineral-oil treatment of sweet com for earworm 
 control. U.S. Dept. Agr., Cir. 667, 15 pp. 
 illus. 
 
 (2) 
 
 I9U3 Styrene dioromide as a substitute for pyrethrum in 
 
 oil for earworm control. Jour. Been. Snt. 36*33&-332. ° 
 
 (3) and Shull, W. S. 
 
 (n.d.) Control of the corn earworm in seed sweet corn. 
 Idaho Agr. Col. War Cir. 20, U pp., illus. 
 
 (U) and Wilcox, J. 
 
 19*+U. Styrene dihromidaJ A substitute for pyrethrum in 
 insecticidal oil used for control of earworms in 
 sweet corn. U. S. Bur. Eat. and Plant Qaar. 25-619, 
 3 pp. (Processed.) 
 
 (5) Davidson, B, H. 
 
 I9U3. The relative effectiveness of some corn earworm control 
 measures in sweet corn. (Scientific Note) Jour. Scon. 
 Snt. 36:936. 
 
 (6) Ivy, s. s. 
 
 19^U. Tests ;.lth DDT on the more important cotton insects. 
 (Scientific Note) Jour. Scon. Snt. 37:1^2. 
 
 (7) Johnson, G. 7. 
 
 19*+h. DDT for control of the tomato fruitworm. (Scientific 
 Note) Jour. Scon. Snt. 37:151-152. 
 
 (2) Stevenson, W. A., Sheets, L. , and Breazeale, J. M. 
 
 19^-. Tests with DDT against pentatomids, mirids, the bollworm, 
 and the cotton aphid. (Scientific Note) Jour. Scon. 
 Snt. 37:1^3. 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 09238 7330 
 
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